Signaling system



Oct. 6, 1936. c. s. DEMAREST SIGNALING'SYSTEM 1o Sheets-Sheet 1 Filed Feb. 1, 1935 INVENTOR 6. S. Dem 0420652 AKFTORNEY 3 c. s. DEMAREST v 5 S IGNALING SYSTEM Filed Feb. 1, 1935 10 Sheets-Sheet 2 INVENTOR C. Sfiezmwucest ATTORNEY Oct. 6, 1936. c. s. DEMAREST SIGNALING SYSTEM Filed Feb. 1, 1935 10 Sheets-Sheet 3 INVENTOR (llflenmlws BY wwm ATTORNEY Oct. 6, 1936.

C. S. DEMAREST SIGNALING SYSTEM I935 10 Sheets-Sheet 4 Filed Feb. 1

INVENTOR 6'. 5. BZIflM/CSZ ATTORNEY Oct. 6, 1936.

C. S. DEMAR EST S IGNALING SYSTEM Filed Feb. 1, 1935 10 Sheets-Sheet 5 INVENTOR C. SEEM Lanai BY fi v - ATTORNEY ESk 98% Oct. 6, 1936. c. s. DEMAREST SIGNALING SYSTEM 10 Sheets-Sheet 6 Filed Feb. 1, 1935 INVENTOR 6. Sflenmms ATTORNEY Oct. 6, 1936. c. s. DEMAREST SIGNALING SYSTEM 10 Sheets-Sheet 7 Filed Feb. 1, 1955 INVENTOR C. Sfiznmxs ATTORNEY Oct. 6, 1936. c. s DEMAREST SIGNALING SYSTEM Filed Feb. 1, 1935 10 Sheets-Sheet 8 INVENTOR 5. S. fienaomes/z ATTORNEY Oct. 6, 1936. c. s. DEMAREST S IGNALING SYSTEM 10 Sheets-Sheet 9 Filed Feb. 1

INVENTOR C. 5. figm/aiaesi ATTORNEY OcL G, 1936. c. s. DEMAREST SIGNALING SYSTEM lOSheets-Sheet 10.

Filed Feb. 1, 1955 INVENTOR I Cflfieim/al eszf ATTORNEY Patented Oct. 6, 1936 UNITE STARS PATENT OFFICE SIGNALING SYSTEM Application February 1, 1935, Serial No. 4,582

14 Claims.

This invention relates to transmission sys-- tems and more particularly to improved means for signaling over such systems.

As the invention is particularly adapted for use in signaling over certain types of long distance telephone circuits, it will be specifically described herein with respect to the type of toll circuit termed a ring-down toll circuit. In the ring-down type of toll circuit, the outgoing toll operator has direct supervision of the calling party only, whereas the distant or incoming toll operator retains the supervision of the called party. While the invention is described with respect to this type of toll circuit, it is understood that it is for the purpose of illustration only and that the arrangements of the invention may be used with other types of transmission systems.

The signaling and control arrangements in toll circuits heretofore used employ direct current, low-frequency alternating current, or voice-frequency alternating current or any combination of these which may be necessary to operate electromagnetic relays and other devices requiring mechanically moving. parts for contact-making and contact-breaking devices either directly or with the aid of associated apparatus. Such apparatus requires mechanical adjustment which sometimes necessitates considerable maintenance in practice. The speed of operation of mechanical devices is limited by the Weight of the moving parts or by the time required to build up a magnetic field in the electromagnetic field. This may limit the application of such devices because of the slowness of operation. In some cases, marginal operation is employed to perform certain operations. The number of margins which are permissible when electromagnetic devices are employed is limited. It is difficult to devise electromagnetic devices which will operate successfully at any but relatively low frequencies. This factor limits the possible uses of various frequencies as means of signaling over telephone circuits. The signaling, and control arrangements now available for telephone systems, therefore, are somewhat limited in the number of variables which may be employed to perform various necessary functions.

In the arrangements of this invention, gasfilled discharge tubes together with circuits designed therefor are utilized to replace all electromagnetic devices. are capable of response to currents of relatively high frequencies as well as low frequencies and Gas-filled discharge tubes direct current. Accordingly, the combination of gas-filled tubes and circuit arrangements therefor result in a signaling system applicable to transmission circuits which is superior to the relay arrangements of the prior art, since the number of frequencies which may be employed and, consequently, the variety of functions obtainable is greatly increased. Moreover, in accordance with this invention, the gas-filled tubes and circuit arrangements therefor are of such design that the tube can be caused to operate or release as the circuit may require by means of the application of carrier frequency, alternating current, alternating currents of lower frequency or direct currents of the proper polarity. The direct current may be supplied by either the primary or secondary cells through resistance units or alternating current which has been rectified by means of rectox units, vacuum tube or gas-filled rectifiers or by any other rectifying device, several of which are well known in the art. Furthermore, in accordance with the arrangements of this invention, more precise and constant specific time of operation is obtained through the use of gas-filled tubes and 25 their associated circuits than can be obtained by using a system of electromagnetic relays. Also, the gas-filled tube circuits are capable of being faster acting than the electromagnetic equipment. The gas-filled tube, and circuit therefor, adapts itself readily to marginal operation. The invention may be more fully understood from the following description together with the accompanying drawings, in the Figures 1, 2, 3, 4, 5, 6, 7,8,1, 2, 3, 4, 5, 6, 7 and 8 of which 0 are shown circuit diagrams embodying one form of the invention. Fig. l is a circuit diagram showing a local subscribers line connecting. the subscriber to a local office. In the drawings, Fig. 2 should be placed to the right of Fig. 1. 40 Fig. 2 shows a cord circuit for the operator at the local office. In the drawings, Fig. 3 is placed to the right of Fig. 2. Fig. 3 shows a recording trunk for interconnecting the local oiilce with a toll oifice. In the drawings, Fig. 6 is placed to the right of Fig. 3. Fig. 6 shows a cord circuit for the operator at the toll ofiice. In the drawings, Figs. 7 and 8 are placed to the right of Fig. 6. Figs. 7 and 8 show the terminal apparatus associated with the toll line at one office. Figs. 4 and 5 show a toll switching trunk when Fig. 5 is placed on the right of Fig. 4. This toll switching trunk may be used to establish a connection between a local subscribers line such as shown in Fig. 1 and a toll operators cord circuit such as shown in Fig. 6. At the distant end of the line, the Figs. 7 and 8 show terminal equipment associated with the toll line at the distant station. Fig. 6 is placed to the right of Fig. 7' and illustrates a cord circuit for the toll operator at the distant toll station. Figs. 4 and 5' with 4 placed to the right of Fig. 5 illustrate a toll switching trunk for establishing a connection from the distant toll oflice to the local office. Fig. 3 is placed to the right of Fig. 6 and shows a recording. trunk for connecting the toll operators cord circuit to the local office. Fig. 2' is placed to the right of Fig. 3' and shows the local operators cord. Fig. 1 is placed to the right of Fig. 2 and shows the subscribers circuit at the distant end of the connection. As Figs. 1', 2, 3, 4', 5, 6, 7', and 8' are identical with Figs. 1, 2, 3, 4, 5, 6, 7 and 8, respectively, similar reference characters with the added designation of prime have been utilized to denote like parts in corresponding figures.

While this invention has been described in one particular form, it is understood that the invention is not limited to the particular arrangement here described, but includes other and similar arrangements within the spirit and scope of the claims hereinafter made.

A brief description of the general operation of the circuits shown will now be given. When the subscriber lifts his receiver from the hook, a lamp appears at the local A operators position. The operator plugs into the subscribers line jack and determines the destination of the call. If the call is to be a toll call and is to pass over a ringdown toll line, the operator plugs into the proper twowire recording trunk to the nearest toll oflice. This lights a lamp in front of the toll operator. She plugs into the recording trunk with a toll cord and determines the destination of the call. Then the toll operator plugs into the proper ringdown toll line with another toll cord and also plugs into a ringdown toll switching trunk back to the B board at the local office of the calling subscriber. This lights a lamp in front of the B operator. The toll operator instructs the local B operator to go into the subscribers line jack over the busy test. The subscriber is now connected to the toll operator by means of the ringdown toll switching trunk and the toll operator pulls down from the two-wire recording trunk, thus giving a disconnect signal tothe local A operator who disconnects the subscribers cord from both the subscribers line and the two-wire recording trunk. The toll operator must ring the distant toll operator by operating her ringing key in the proper direction. When she does this, a. lamp appears in front of the distant toll operator, who answers by plugging a toll cord into the line jack. The distant operator, aft-er learning the destination of the call, plugs into the proper ringdown toll switching trunk. This lights a lamp in front of the local B operator who answers the call. The called number is passed from the toll operator to the local B operator who connects the called partys line to the ringdown toll switching trunk. The toll operator receives an indication that the subscribers line has been reached and then rings the subscriber.

When the calling subscriber hangs up, a disconnect lamp appears before the outward toll operator. She then disconnects from the toll switching trunk, causing a lamp to light in front of the local B operator who disconnects from the subscribers line. At the called end, a lamp appears before the toll operator, indicating that the called party has hung up. She also disconnects from the toll switching trunk and a. signal appears before the B operator who pulls down from the called subscribers line. If the toll call has passed over more than one toll link, it is the duty of the outward toll operator to clear the circuit by ringing the various intermediate points. In doing so, a lamp appears before the intermediate operator and she is instructed to clear the circuit.

In the following detailed description of the invention the word operate or derivatives thereof will be used hereafter to indicate the process which takes place within the gas-filled tube when the gas contained inside the bulb ionizes and an arc is established from the cathode to the anode. The word release or derivatives thereof will be used hereafter in the description of this invention 1 to indicate the extinction of the arc and the return of the gas to a deionized condition. Each of the gas-filled tubes in this invention has a protective resistance I in the plate circuit of the tube. This protective resistance is used to limit the current in the plate circuit to a suitable or a safe value. Furthermore, each gas-filled tube has a protective resistance 2 in the grid circuit in order to prevent the grid current of the tube from becoming excessive. anoes are to be found associated with every tube, no further mention will be made of their purpose.

The type of gas-filled tube illustrated herein is of the heater type and the heater power is shown supplied by means of direct current. It is understood in this invention that the cathode is not necessarily of the heater type and the powor supply is not limited to direct current. Furthermore, the gas-filled tube may be of the cold cathode type, under which conditions no heater or heater power would be required.

The types and characteristics of the gas-filled tubes used in this invention will be varied and numerous and each gas-filled tube employed will be of the type and have the characteristics which are essential and necessary toperform the func- Since these resisttions which are required of the tube by the use to which it is specifically put.

One type of tube is of such physical dimensions that it is capable of being mounted in the switchboard in front of the operator. The gas content of this tube is of such a nature and pressure that a visual glow is present when the tube is operating. However, this type of tube may be used not only to give a visual indication but also perform some electrical function such as the completion of a circuit. For example, an electromagnetic relay might have its Winding included in the plate circuit of the tube in place of the protective resistance I heretofore referred to and might be utilized under control of the tube for any desired purpose. This type of gas-filled tube will be referred to in this description as a combined lamp and relay.

Another type of tube has five electrodes contained within the envelope. The cathode, grid and anode perform the same functions as in the three-element tube. In addition to these elements are two probes or plates. When the tube is in the non-operate or release condition, the resistance between these two probes or plates is very high but when the tube operates the resistance between these two probes or plates is relatively low. Hence, by properly connecting such a tube in the circuit, the circuit may be completed through these probes or plates. This type of tube will be referred to in this description as a five-element tube.

The other types of tubes employed in this invention are of such physical characteristics and have such gas and gas pressure within the envelope as to meet the requirements which are set up by the circuit conditions.

In the embodiment of the invention described herein, certain gas-filled tubes are arranged for self-quenching by means of an alternating potential applied to the anode circuit from a separate source of alternating current. It is of course, within the scope of this invention to apply other quenching means to such tubes as may require the same in the practice of this invention. For example, the aforesaid tubes may be arranged to oscillate in such a manner that the plate current is interrupted once for each cycle of the operation. It would also be possible to provide auxiliary quenching tubes in such a manner that the discharge would be transferred from one of the main tubes to the auxiliary tube when required.

Certain gas-filled tubes described in this invention are so connected as to be associated with an auxiliary circuit. Connection is made to these auxiliary circuits through a transformer connected in the plate circuit of certain gas-filled tubes. The words auxiliary circuit are taken to mean such equipment as may not be required if the telephone circuit is to include the connections required for connecting together and giving service to only two subscribers, one at each end of the toll connection or local connection. This auxiliary circuit equipment would be necessary if more than the above mentioned subscribers were to be served. It is proposed in this description to indicate a method or methods of connecting such auxiliary equipment to the herein described telephone circuit.

Figure 1 is the subscribers line circuit. Gasfilled tube T1 is a combined lamp and relay. The operation of tube T1 is under the control of the subscribers switchhook and tube T2, which is controlled by the sleeve circuit of jack I4, 25 or 26. When the subscriber lifts the receiver 3 from the switchhook 4, the subscribers line is shortcircuited through one of the coils of the induction coil 6 and the telephone transmitter 5. This short-circuit on the line applies the positive potential of battery l to the grid of tube T1. The applied potential is of such a polarity as to cause the tube to operate. Tube T1 is normally held released by the negative potential of battery 8 applied to the grid of tube T1 through the resistance 9. The plate of tube T1 is supplied with an alternating potential by generator in through transformer ii. The operation of the tube T1 gives a visual signal to the operator and also allows plate current to flow in the plate circuit of the tube. This current induces a voltage in the secondary of transformer i2 which supplies an auxiliary circuit I3, the purpose of which has been discussed previously. Anti-resonant circuits 2! and 22 con-,

sisting of suitable coils and condensers are placed in the input circuit of tube T1 to prevent the alternating current which is supplied to the plate of the tube from being fed back into the telephone line. When the local A operator answers the call, she plugs into the subscribers line jack I4. This applies negative battery to the sleeve of the jack. The potential drop across resistance 95 in the sleeve circuit is such that tube T2 operates. When tube T2 operates, potential is supplied to rectifier [9 by transformer H5. The plate supply of tube T2 is supplied by alternator i1 through transformer l8. Tube T2 is held normally unoperated by a negative potential supplied by battery 20. Unidirectional current supplied by rectifier l9 flows through filter 23 to resistance 24. The potential drop across resistance 24 is in such a direction as to apply the proper polarity to the grid of tube T1 to cause it to release, thus extinguishing the visible glow. Whenever a cord is plugged into any of the subscribers line jacks, I4, 25 or 26, switchhook signals from the subscriber are transferred to the cord circuit and do not cause tube T1 to operate. Jacks 25 and 26 are at inward positions or at the local B operators position and the insertion of a plug in these jacks causes tube T2 to operate and function in the circuit as described above. When the subscriber hangs the receiver 3 on hook 4, the short circuit is removed from the subscribers line and the potential of battery 1 is removed from the grid of tube T1. Hence, when the cord is pulled out of either jack, I4, 25 or 26, tube T1 will not operate and no visible signal is given to the local A operator by tube T1. Should a cord be pulled out of either jack I4, 25 or 26, before the subscriber has placed his receiver on the hook, tube T1 would operate and a visual signal would appear before the local A operator.

Figure 2 is the local operators cord circuit which is available to the local A operator. The subscribers cord circuit is used toanswer calls from the calling subscriber and connect the subscriber to the toll operator if the call is to be a toll call. Incoming calls are answered by inserting plug 27 into the subscribers line jack. This operation completes the sleeve circuit of the plug, thus allowing a current to iiow from battery 29 through resistance 28. The completion of the sleeve circuit applies a negative potential to the grid of tube T3 of such a value that the tube is released. Tube T3 is normally held operated by the positive potential of battery 30 being applied to the grid of the tube. Alternating potential from generator 3! is applied to the plate of tube T3 through transformer 32. When tube T3 is operated, an alternating potential is applied to rectifier 33 through transformer 34. The unidirectional voltage from rectifier 33 passes through filter 35 and is applied toresistance 36. The potential applied across resistance 36 is in such a direction as to oppose the potential supplied by battery 3i to the grid of tube T4. The potential supplied to the grid of tube T4 from battery 3'! is applied thereto through resistances 42 and 36. Tube T4 is accordingly normally re leased. When the subscribers cord is plugged into a line jack tube T3 will release as heretofore pointed out and this will tend to cause tube T4 to operate. However, this operation of tube T4 at this time will be prevented if the subscribers receiver is off the hook as will be pointed out hereinafter. The tube T4 is a combined lamp and relay and is so placed as to be visible to the operator. The alternating potential for tube T4 is supplied by generator 38 through transformer 39. Another transformer is used to connect the subscribers cord circuit shown to auxiliary apparatus, the purpose of which has been discussed above. When the subscribers receiver is off the hook, current from battery 46 is supplied to the subscribers line through the repeating coil 4|. This current flows through resistance 42 in such a direction that the potential drop across this resistance is in opposition tothe potential applied to the grid by battery 31 and hence will cause a negative potential to be applied to the grid of tube T4. thus releasing the tube. Antiresonant circuits 43 are employed to prevent the alternating potential which is applied to the plate of tube T4 from feeding back into the subscribers line circuit. When the subscriber places the receiver on the hook, current ceases to flow through resistance 42 and tube T; will operate, thus giving a visual signal to the operator that the subscriber has completed the call. When the operator pulls plugs 21 out of the line jack, tube T3 operates due to the ceasing of the current flow through resistance 28. When tube T3 operates, a potential is applied across resistance 36 in such a direction as to cause tube T4 to be released. The tubes T and Ta and the equipment associated with them perform in a manner substantially similar to that described immediately above for tubes T3 and T4. This will be pointed out in more detail hereinafter. Key 44 is the key by which the operator can connect her telephone to the line circuit. The o-perators telephone set and associated equipment are connected at 45. Key 46 is a ringing key to apply ringing current to the subscribers line should the A operator desire to call a subscriber. Condensers 41 are used to send audible ringing current back to the calling subscriber.

Figure 3 is the two-wire recording trunk which connects the local A operator to the toll operator. This is the trunk over which information is passed from the calling subscriber through the local operator to the toll operator. The trunk is not used to complete the call but the toll operator reaches the calling subscriber over a different type of trunk and the calling subscriber communicates with the called subscriber over the last mentioned trunk but not over a two-wire recording trunk. Signals are transmitted over this trunk only from the local A operator to the toll operator.

The local A operator reaches the toll operator by inserting the plug of the subscribers cord circuit into jack 48. Resistance 49 completes the sleeve circuit of this jack to ground. The plug of a subscribers cord circuit is so wired that the tip has a positive and the ring a negative potential on it. The positive potential applied to the tip of the trunk places a positive potential on the grid of tube T600 causing it to operate. Tube T600 may be the type of combined lamp and relay which has been referred to previously. Tube T600 is normally held released by the potential of battery 50 applied to the grid of the tube through resistance 5!. The plate potential of tube T600 is supplied by generator 52 through transformer 53. A transformer is used to connect this circuit with auxiliary apparatus. Anti-resonant circuits 56 prevent the alternating potential applied to the plate of the tube from going out to the line. When tube T600 operates, there is a visual indication to the toll operator that a call is waiting on that trunk. She inserts the plug of a toll cord into the jack 5?.

The sleeve circuit of jack 5'! contains inductances 58 and resistances 59, 69 and BI. When the plug of the toll operators cord circuit of Fig. 9- is inserted into the line jack 5?, current flows from battery 293 through resistance 60 in such a direction as to apply a positive potential to the grid of tube T7, causing it to operate. Tube T1 is held normally released by the negative potential supplied tothe grid of this tube by battery 68. Generator 62 supplies the alternating plate potential of the tube through transformer 63. When the tube operates, alternating current is applied to rectifier 64 through transformer 65. The unidirectional current from rectifier 64 flows through the filter 66 into the resistance 61. The resulting potential across resistance 61 is in such a directon as to oppose the potential applied to the tip and ring of the line circuit by the subscribers cord circuit. This releases tube T600- When tube T1 operates,alternating potential is applied to the grid of tube Ts through transformers 65 and 69. The tube T8 is held normally released by the negative potential supplied by battery 10. When tube Ts operates, it applies a relatively low impedance across the tip and ring of the two-Wire recording trunk through the anti-resonant circuits H. The placing of this low impedance across the tip and ring conductors of the recording trunk will allow current to flow from battery 49 through a resistance in the local A operators cord circuit similar to resistance 42, which in a manner similar to that described with respect to resistance 42 will apply a potential to tube Ts so as to extinguish or release it. These anti-resonant circuits are placed in the leads to the trunk to prevent the alternating potential supplied to the tube by generator 12 through transformer 13 from being fed back into the trunk. When the toll operator has received the instructions concerning the call, she pulls down from the line jack 51. This removes battery 293 from the sleeve of the jack and thus removes the potential drop across resistance 60. Tube T1 releases and removes the potential drop from across resistance 61 and also removes the voltage from the grid of tube T8 causing it to release. The removal of the potential drop across resistance 61 will cause tube T600 to operate at this time and it will remain operated until the jack is removed from plug 48. The release of tube T8 removes the low impedance from across the two-wire recording trunk. This gives a visual indication to the local operator to remove her plug from the line jack of the recording trunk. When the local operator removes the subscribers cord from the recording trunk, tube T600 is extinguished as referred to above.

The operation of tube T9 will now be described. When the local operator inserted the subscribers cord in the jack of the two-wire recording trunk, a positive potential was placed on the grid of tube T9, causing it to operate. Tube T9 is .normally held released by the negative potential supplied by battery 74 through resistance 15. The plate potential of tube T9 is supplied by the generator 16 through transformer 11. When an alternating current flows in the plate circuit of tube T9, an alternating potential is applied to rectifier unit 19 by transformer 18. The unidirectional current from rectifier 19 flows through resistance 59 in such a direction as to oppose the fiow of current caused by the battery 293 in the sleeve circuit. The value of this potential, however, is not sufliciently great to prevent the operation of tube T7, but it is of a sufiicient value to make the sleeve circuit appear as a very high impedance circuit to the toll cord. Should the cal operator remove the plug from jack 48 before the toll operator removes the plug from jack 5'1, tube T9 would be released, the counter voltage set up by the rectifier unit 19 would be removed, and the sleeve circuit would appear as a low impedance circuit. This would cause a visual signal at the toll operators position in the toll cord circuit due to the operation of tube T43 as hereinafter described. Anti-resonant circuits 80 are placed in the input circuit of tube T9 to prevent the alternating potential supplied to the plate of that tube from feeding back into the two-wire recording trunk.

Figures 4 and 5 are the tool switching trunk circuits which are used to complete an outgoing or incoming call which is to be made over a ring down toll line. These switching trunks are so designed that the supervision of the subscriber is transferred to the toll operater at the outward position. As has been heretofore pointed out the toll operator has connected her cord circuit, shown in Fig. 6, with the recording trunk of Fig. 3. When she ascertains what the long distance call is to be she will now plug into a toll switching trunk, such as shown in Figs. 4 and 5, with another toll cord circuit identical with that of Fig. 6. Instead of illustrating an additional cord circuit reference will be had to that shown in Fig. 6. Considering Figure 5, when the toll operator plugs into the line jack 8! with a plug such as 289 of toll cord such as shown in Fig. 6, battery 293 is supplied to the sleeve circuit such that the potential drop across resistance 82 is in the proper direction to cause tube T10 to operate. Tube 10 is normally held unoperated by the negative potential supplied by battery 83 to the grid of the tube. Resistance 84 in the sleeve circuit is a protective resistance. Alternating potential is supplied to the plate of tube T10 by the generator 85 through transformer 85. When tube T10 operates, an alternating potential is supplied to the grid of tube T11 by means of transformer 8?. Tube T11 is held normally released by the negative potential supplied by battery 08 to the grid of the tube. Alternating potential is supplied to the plate of tube T11 by generator 89 through transformer 90. When tube T11 operates, a short circuit is applied to the tip and ring of the toll switching trunk through anti-resonant circuits 9!. These anti-resonant circuits are used to prevent the alternating potential from generator 89 from being fed to the tip and ring of the trunk.

Referring to Fig. 4, the short circuit applied to the tip and ring of toll switching trunk by the operation of tube T11 causes the current from battery 96 to flow through inductances 93, resistances 94 and 95. When current flows through resistance 95, the potential drop across the resistance is in such a direction as to cause the operation of tube T12. Tube T12 is held normally released by the potential supplied by battery 91. Alternating potential is applied to the plate of tube T12 by generator 98 through the threewinding transformer 99. This transformer also supplies the plate potential of tube T13. When tube T12 operates, an alternating current flows through the plate circuit. This causes current to fiow through the primary winding II3 of the three-winding transformer I00. Alternating potential from the secondary of this transformer is applied to rectifier unit Illl and a unidirectional potential results across resistance I02 which is supplied from this rectifier through filter I03. The unidirectional potential resulting across resistance I02 is in such a direction as to cause the operation of tube T11. Tube T14 is a combined lamp and relay of the type which has been described above. Generator I04 supplies the alter nating potential for the plate of tube T14 through transformer I05. Transformer I06 is inserted in the plate circuit of the tube T11 for the purpose of relaying signals to auxiliary apparatus. Tube T11 is held normally unoperated by the negative potential supplied to the grid of the tube by battery I07.

When tube T14 operates, it gives a visual indi cation to the local B operator that there is a call on that particular toll switching trunk. The local B operator connects her telephone set to the trunk by means of key I08. After she has been instructed as to the number called, she inserts plug I09 into the proper subscribers line jack. This completes the sleeve circuit of plug I09. Battery I I0 causes current to fiow through resistance I I I. The potential drop across resistance III is in such a direction as to cause the operation of tube T13. This tube is held normally released by the negative potential supplied to the grid of the tube by battery II2. When tube T12 operates, current is caused to fiow through primary winding II4 of the three-winding transformer I00. Since the plate potential of tube T13 is supplied by generator 98, the relative phases of the currents flowing through windings I I3 and H4 are so adjusted as to result in no potential across the secondary of this transformer when current is flowing in both primary windings. When there is no alternating potential across the secondary of transformer I00, tube T14 is released.

Transformer I I5 located in the plate circuit of tube T13 supplies an alternating potential to rectifier I I6. This causes a unidirectional potential across resistance I I! of such a polarity as to cause tube T15 to operate. Filter H8 is inserted between the rectifier and the resistance I I1. Tube T15 is held normally released by the negative potential supplied to the grid of the tube by battery H9. When tube T15 operates, an alternating potential is supplied by transformer Ml to rectifier I20. This is in such a direction as to add to the potential supplied to the tip and ring of the toll switching trunk by battery 96. Generator I2I supplies the plate potential of tube T15 through transformer I22.

Returning to Fig. 5, tube T16 is so adjusted as to remain unoperated until the added potential supplied by rectifier I20 increases the potential difference between the tip and ring of the toll switching trunk to such a value that tube T16 operates. This tube is normally held unoperated by the potential of battery I23 applied to the grid of the tube through resistance I24 to the grid of the tube. Generator I25 supplies the plate potential for tube T16 through transformer I26. Anti-resonant circuit I21 is inserted in the input circuit of tube T16 to prevent the alternating potential applied to the plate from being applied to the toll switching trunk. When tube T16 operates, an alternating potential is applied to the grid of tube T11 through transformer I29. Tube T11 is normally held released by the negative potential of battery I29. Alternating potential is supplied to the plate of tube T11 by generator I through transformer I3I. The purpose of tube T11 is to change the sleeve impedance of the toll cord. This sleeve impedance is changed from a high value to a low value by the operation of tube T11. This causes a visual signal to appear before the toll operator in a manner tobe pointed out in more detail hereinafter. This signal is an indication that the subscribers line has been reached.

The subscribers bell is rung by the toll operator. She does this by pulling a ringing key in the toll cord circuit which applies a negative potential to the tip of the toll cord and the toll switching trunk. When the toll operator pulls the ringing key, a current flows through resistance I32 in such a direction as to cause the operation of tube T13. Tube T13 is normally held released by the negative potential supplied by battery I33. Tube T13 is supplied with plate potential by generator I34 through transformer I35. Anti-resonant circuit I36 prevents the alternating current supplied to the plate of tube T13 from being fed back into the talking circuit. Anti-resonant circuit I31 is resonant to the ringing frequency which is used to ring over the toll switching trunk. When the tube T12 operates, ringing current supplied by generator I38 through transformer I39 is applied to the line through coils I40. Circuit I42 is an anti-resonant circuit, resonant to the frequency of the alternating potential supplied by generator I34 to the plate of the tube.

When tube T13 operates it not only supplies ringing current to the line but it also extinguishes tubes T10 and T20. These tubes cut off the toll operator while ringing current is being sent out over the line. Since both of these tubes operate in the same manner, the operation of only one will be described. Considering tube T10, alternating current is supplied to the plate of this tube by generator I43 through transformer I44. Anti-resonant circuits I45 are inserted in the tip and ring of the toll switching trunk to prevent the alternating current of the frequency supplied to the plate from getting out on the line. Tube T13 is normally held operative by the positive potential of battery I46 applied to the grid through resistance I41. When tube T13 supplies alternating current through transformer I48 to transformers I49, tubes T10 and T20 are extinguished. Alternating potential is applied to rectifier unit I50 and unidirectional potential is applied to resistance I41 through filter I 5| The potential drop across this resistance is in such a direction as to cause tubes T10 and T20 to release. The release of these tubes breaks the toll switching trunk at this point.

When the ringing current passes over the toll switching trunk, it passes through condensers I52 and through transformers I53, causing the release of tubes T21 and T22 and the operation of tube T23. The circuits of tubes T21 and T22 are identical with those of tubes T13 and T20 and the operation will be omitted as it is described in detail for tube T13. Tube T23 is caused to operate by the application of an alternating potential to the grid of that tube due to the ringing current from source I38. The tube T23 is held normally released by the negative potential supplied by battery I60. Generator I62 supplies plate potential to tube T23 through transformer I63. When tube T23 operates, ringing current supplied by generator I64 is applied to the subscribers line through transformers I65. Anti-resonant circuit I66 prevents the alternating current of the generator I62 from being applied to the subscribers line.

When the subscriber lifts the receiver from the switchhook, a short circuit is applied to the subscribers line and this short circuit causes a current to flow from battery I68 through resistance I61. The potential drop across resistance I61 is of such a polarity as to cause the release of tube T15 which had been caused to operate by the operation of tube T13 which in turn resulted from the operator inserting the plug of the toll switching trunk into the subscribers line circuit. Antiresonant circuit I69 prevents the alternating potential from the generator I2I from entering the talking circuit. The release of tube T15 will cause the release of tubes T10 and T11 and will allow the sleeve circuit of the trunk to resume its initial high impedance, thereby releasing tube T43.

When the subscriber completes the conversation and places his receiver on the hook, the short circuit is removed from the subscribers line and current ceases to flow through resistance I61. Since plug I09 is still inserted in the line jack, tube T13 is operated and hence tube T15 will again operate due to the potential drop across resistance II1. When tube T15 operates, the added potential set up by rectifier I20, as explained above, is applied to the potential of battery 96. This increase in potential results in the operation of tube T10 in Fig. 5. The operation of tube T10 reduces the resistance in the sleeve circuit of jack 8I and causes a visual signal to be given in the toll cord circuit to the toll operator indicating that the subscriber has hung up. When the toll operator pulls down from jack 8|, tube T10 releases, and, in doing so, releases tube T11 which removes the low resistance circuit from the toll switching trunk. This causes the release of tube T12 of Fig. 4. When tube T12 releases, no current flows through winding II3 of transformer I00. Since the plug I09 is still inserted in the subscribers line jack, tube I113 is operated and current flows through Winding I I4 of transformer I00. With this condition of currents in transformer I00, namely, only current flowing through one primary, tube T14 operates, giving a visual signal to the local B operator and instructing her to pull down from the subscribers line circuit. When the local B operator disconnects the plug I 09 from the line jack, tube T13 releases. This removes all the potential from the secondary of transformer I00, thus releasing tube T14. The circuit as a Whole is then restored to its normal condition.

Fig. 6 is a circuit diagram of a toll cord used by the toll operator to complete toll connections. Tubes T43 and T44 are combined lamp and relays. Since these tubes are identical in operation, only tube T43 will be described. Tube T43 is held normally released by the negative potential of battery 284 which is applied to the grid. Generator 285 supplies the alternating potential for this tube through transformer 286. The toll cord is so arranged that an on-hook signal is transmitted to the toll cord as a high impedance sleeve. An off-hook signal is transmitted to the toll cord as a low impedance sleeve. A high impedance sleeve circuit will result in the flow of very little current through resistance 281 from battery 293. This will not cause tube T43 to operate. However, when the sleeve impedance is reduced, current flows from battery 293 through resistance 281 and the potential drop across this resistance is of such value and in such a direction as to cause the operation of tube T43. Key 288 is used by the toll operator to connect her head set to the toll line. In operating from the normal to the operating condition this key momentarily breaks the sleeve circuit of plug 289. This is necessary to cause certain operations in associated equipment into which plug 289 may be inserted. Key 290 is a ringing key by means of which the toll operator may ring ineither direction. This key applies a negative potential from batery 2! through resistance 292 to the tip of the toll cord.

Consideration will now be given to the description of the toll terminal equipment. Fig. 7 is a circuit diagram of the terminal equipment used to complete the type of call termed ringdown as previously described. This terminal equipment is so designed that the outward toll operator must ring the distant toll operator by operating the ringing key in the toll cord circuit. Incoming calls from the distant end are received by the ringer shown in Fig. 8 (enclosed in dotted lines) and detected therein. The detected ring is transferred to the equipment shown in Fig. 7 and results in a visual signal being given by the combined lamp and relay T52. After a toll connection has been established between two toll operators all subsequent rerings appear in the toll cord circuits such as shown in Figs. 6 and 6. In this type of circuit, there is no automatic signal which indicates that either operator has pulled down from the circuit but one operator must ring the other stating her intention to pull down.

When the toll operator at the toll office at which the cord of Fig. 6 is located desires to make a call over the ringdown toll line she inserts the plug of the toll cord into a jack such as 300. This completes the sleeve circuit of the jack. The resulting potential drop across resistance 362 is of such a value and in such a direction as to cause the operation of tube T45. This tube is held normally released by the negative potential supplied by battery 363 to the grid of the tube. Generator 304 supplies an alternating potential to the plate of tube T45 through transformer 365. When tube T45 opcrates voltage is applied through transformers 366 and 308 to the rectifier circuit consisting of rectifier 309, filter 3H] and resistance 3. The unidirectional potential drop across resistance 3!! is of such a value and in such a. direction as to make the resulting voltage on the plate of tube T45 insufficient to maintain the tube operated. Battery 3H2 supplies the normal plate potential to the anode of tube T45 and the potential to the probes of said tube through the inductance 3666 which is of a suflicient value to cause the line to be essentially opened at such time as the said tube is unoperated. The tube is held normally operated by the positive potential of battery 3l3 applied to the grid. This tube is of the five-element type referred to heretofore. One of the probes or plates is connected to one side of the line through resistance 3M and condenser 3|5 and the other probe is connected to the other side of the line through a similar resistance and condenser. When tube T45 is operated, the toll line is terminated by the aforementioned resistances and condensers and when the tube is released the termination is removed. It is the practice in telephony to terminate ringdown toll lines in a network comprising resistance and capacitance for well known transmission purposes.

When the toll operator rings out over the toll line, she does so by pulling the ringing key in the toll cord circuit. This applies a negative potential to the tip of the toll line. This operation causes the operation of tube T49 which applies signaling current to the toll line and causes the release of tubes T50 and T51 which disconnect the toll line from the toll operator during the ringing period. Tube T49 is normally held released by the negative potential of. battery 326 applied to the grid of the tube through resistance 321. Generator 329 supplies alternating potential to the plate of the tube through transformer 330. Anti-resonant circuits 328 and 33! prevent alternating current of the frequency applied to the plate from being fed into the toll line. When tube T49 operates ringing current is supplied to the toll line from generator 332 through transformers 333 and 334.

When tube T49 operates an alternating potential is supplied to rectifier 335 which applies a unidirectional potential across resistance 336 of such a value and in such a direction as to result in the direction of the plate potential of tube T50 below that value which will maintain the operation of the tube. Battery 336 normally supplies the plate potential of tube T50. The positive potential of battery 339 applied to the grid of the tube keeps it normally operated. Tube T51 operates in an identical manner and the method of operation will not be repeated. The transmission of ringing current over the toll line will result in the operation at the distant end of ringing equipment similar to that of Fig. 8 and shown at Fig. 8' at the distant end of the line. This will result in the operation of tube T52 of Fig. 7' which is similar to tube T52 of Fig. 7. This will tell the inward operator that there is a call on the line.

Tube T52 is a combined lamp and relay which operates when potential of. the correct polarity is supplied across resistance 325 by circuit 356 which goes to the output circuit of Fig. 8. Tube T52 gives 5 a visual indication to the toll operator that the distant toll operator is calling her on that particular ringdown toll line. She answers the call by inserting the plug of a toll cord into a jack such as 360. The method of operation and release of this tube will not be described here since the tube operates in a manner similar to that described previously for tube T1, Fig. 1 of the subscribers line circuit.

Fig. 8 is. a circuit diagram. of the ringer associated with the ringdown toll equipment. The ringer is used to detect incoming rings from the distant end of the toll line. The input circuit 351 of Fig. 8 is connected to the directional selection circuits 358 well known in the art and shown on Fig 7.

After the incoming signal passes through the directional selection circuits 358 of Fig. '7, it passes through condensers 346 and resistances 3 H of Fig. 8 to the input transformer 343. The primary of this transformer is tuned to the carrier frequency of the impressed ringing frequency by the resonant circuit 342. The voltage of the incoming signal is impressed on the grid of the tube T53 through the slow operate and release circuit consisting of resistances 3M and 3A5, rectifier 346 and condenser 341. Tube T53 is held normally released by the negative potential of battery 339 applied to the grid through resistance 348. Plate potential is supplied to the tube by generator 356 through transformer 35H. When the tube operates an almrnating potential is supplied to the rectifier unit 355 by transformer 352 through the tuned circuit consisting of inductances 353 and condensers 35 5 which is made resonant to the modulation frequency of the incoming ringing signal. The output circuit 356 of Fig. 8 supplies a unidirectional potential of the proper value and polarity to resistance 325 of Fig. 7.

When a plug of a toll cord is inserted in jack such as 366 the circuit of tube T47 is put in the proper operating condition to care for rerings from the distant toll operator. Whenever the plug of. a toll cord is in jack 366, potential isv supplied to the plate of tube T47 by the plate circuit of tube T through transformer 30'! and rectifier 316. Condenser 3 I! is a filter condenser in parallel with resistance 3i8 across which the output of the rectifier is applied. The potential drop across resistance 3l8 supplies the plate voltage of tube T47. Should the distant toll operator rering, a potential of such a value and polarity is supplied across resistance 325 by the associated ringer over leads 356 that tube T47 operates. Since the plate of this tube is supplied with direct current, the tube remains operated even after the incoming ring has been removed. Tube T47 is kept released before the incoming ring causes it to operate by the negative potential of battery 3 l 9 applied to the grid through resistance 320. When tube T47 operates current flows through resistance 32! in such a direction that the potential drop across the resistance is ofv the correct polarity to cause tube T48 to operate. This tube is held normally released by the negative potential of battery 322. The plate of tube T48 is supplied with an alternating potential from generator 323 through transformer 324. When tube T43 operates, high resistance 30! is short-circuited and the sleeve circuit becomes one of low impedance to ground through the operated tube T48. This change in the sleeve circuit from a high to a low impedance will serve to operate tube T44 in the toll operators cord circuit. This will give a visual signal to the toll operator. When the toll operator answers the visual indication which is thus given in the toll cord circuit by the rering signal, she operates a key which not only connects her headset to the line but also momentarily breaks the sleeve circuit. In breaking the sleeve circuit, tube T45 is momentarily released and hence the plate potential is removed from tube T47 and this tube is released thus releasing tube T48 and removing the low impedance ground from the sleeve and extinguishing the lamp in the toll cord circuit.

As has been heretofore pointed out, when the toll operator at the toll office where the cord circuit of Fig. 6 is located operates her ringing key and transmits ringing current over the toll line, the tube T5z at the distant end of the toll line will be operated and will give a visual signal to the toll operator at that end of the line. The apparatus shown in Figs. l' and 8 at the distant end of the toll line is identical with that shown in Figs. '7 and 8 and accordingly no detailed description of its operation will be given. In response to the operation of the visual signal T52,

the toll operator at the distant end of the line will connect her cord circuit shown in Fig. 6' with the toll line and will ascertain the called number. Fig. 6 is identical with Fig. 6 and no further description thereof will be given. The toll operator will then connect the toll line through her cord circuit of Fig. 6 with a toll switching trunk such as that shown in Figs. 4' and 5'. As Figs. 4 and 5 are identical with Figs. 4 and 5, no detailed description of their operation will be given. The toll operator will now transmit a ringing signal over the toll switching trunk of Figs. 4 and 5'. This will cause the operation of the tube T14 of Fig. 4 and will give a visual signal to the local operator. The local operator may then ascertain the called number and will then connect the toll switching trunk of Figs. 4' and 5' with the subscribers line such as shown in Fig. 1, which is identical with that of Fig. 1.

Obviously, when a call originates at the distant end of the line the toll switching trunk and the local operators cord circuit shown in Figs. 3' and 2' would be utilized. These figures are identical with Fig. 3 and Fig. 2, and no detailed description of their operation will be given.

While the invention has been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood that it is capable of embodiment in many and other widely varied forms without departing from the spirit of the invention as defined by the appended claims.

What is claimed is:

1. In a telephone system including a subscribers circuit and a local operators cord circuit, the combination of signaling means including a first gas-filled discharge tube (T1Fig. 1) normally released and located in said subscribers circuit, means operative when the subscriber seizes the line for use for operating said first tube, a second gas-filled discharge tube T2Fig. 1) in said subscribers circuit normally unoperated and adapted to be operated when said subscribers circuit is connected with said cord circuit, means controlled by the operation of said second tube for releasing said first tube, a third gas-filled discharge tube (Ts-Fig. 2) located in said cord circuit, normally operated and adapted to be released when said cord is connected to said subscribers line, signaling means comprising a fourth gas-filled discharge tube (T4Fig. 2) located in said cord circuit and normally released, means controlled by said third tube when operated for maintaining said second tube released, means operative when said cord circuit is connected to said subscribers line for releasing said third tube, and means operative when a talking circuit is completed over said subscribers line and said cord circuit for maintaining said fourth tube released.

2. In a telephone system including a subscribers circuit, a toll switching trunk, a toll operators cord circuit, the combination of a supervisory gas-filled discharge tube (T14-Fig. 4) capable of giving a visual signal and located in said toll switching trunk at the subscribers end thereof, a first gas-filled discharge tube (T10Fig. 5) in said trunk normally released and controlled by the connection of said toll cord to said trunk, means controlled by the operation of said first tube for closing a low impedance path across the talking conductors of said trunk, a second gas-filled discharge tube (T1zFig. 4) in said trunk normally released, means operable when said low impedance path is closed for operating said second tube, and means controlled by the operation of said second tube for operating said supervisory tube.

3. In a telephone system including a subscribers circuit, a toll switching trunk, a toll operators cord circuit, the combination of a supervisory gas-filled discharge tube (Tm-Fig. 4) capable of giving a visual signal located in said toll switching trunk at the local oifice end thereof, a plurality of gas-filled discharge tubes and circuit arrangements therefor associated with said previously mentioned circuits and operative when said toll cord is connected to said switching trunk to exclusively cause the initial operation of said supervisory tube, and a plurality of gas-filled discharge tubes and circuit arrangements therefor associated with said previously mentioned circuits and operative when said subscribers line is connected to said switching trunk to cause the subsequent release of said supervisory signal.

4. In a telephone system including a subscribers circuit, a toll switching trunk, a toll operators cord circuit, the combination of a supervisory gas-filled discharge tube (Tia-Fig. 6) capable of giving a visual signal located in said toll operators cord circuit, a plurality of gas-filled discharge tubes and circuit arrangements therefor associated with said previously mentioned circuits and operative when a connection is established between all of said circuits for causing said supervisory tube to give a signal, the type of signal given by said supervisory tube being determined by the condition of the switchhook in said subscribers line.

5. In a telephone system including a subscribers circuit, a toll cord having a supervisory gasfilled tube (T43Fi 6) capable of operating a visual signal associated with the sleeve circuit thereof, a toll switching trunk including a first gas-filled discharge tube (Tia-Fig. 4) normally released and operated when said trunk is connected to said subscribers line, a second gas-filled discharge tube (T15Fig. 4) normally released and conditioned for operation by the operation of said first tube, means controlled by operation of the subscribers switchhook for operating said second tube, a chain of gas-filled tubes in said trunk controlled by the operation of said second tube, means controlled by said chain of gas-filled tubes for varying the characteristics of the sleeve circuit of said toll cord when connected to said trunk whereby said supervisory tube may be controlled.

6. In a telephone system the combination of a toll operators cord circuit, means in said cord circuit for applying a potential to the tip conductor of said cord, a toll switching trunk including a gas-filled discharge tube (T1aFig. 5) normally unoperated and operated by the potential applied to the tip conductor of said toll cord when connected to said trunk, means for applying ringing current over said trunk controlled by the operation of said first tube, second and third gasfilled discharge (T19T2o-Fig. 5) tubes normally operated and connected serially in the tip and ring conductors of said trunk at the toll operators end thereof, and means controlled by the operation of said first tube for releasing said second and third tubes.

'7. In a telephone system including a calling operators toll cord, a toll line and a called operators toll cord, the combination of a supervisory gas-filled discharge tube (T52Fig. '7) located in said toll line at the called operators position, and being capable when operated to act as a visual supervisory signal itself, and a plurality of gas-filled discharge tubes and circuit arrangements therefor associated with said toll line and started in operation by the application to said toll line of ringing current by the calling operator, said plurality of gas-filled tubes being arranged to cause the operation of said supervisory tube.

8. In a telephone system including a calling operators toll cord, a toll line and a called operators toll cord, the combination of a supervisory gas-filled discharge tube (T'5zFig. 7') capable of operating a visual signal located in said toll line at the called operators position, a first chain of gas-filled discharge tubes and circuit arrangements therefor associated with said toll line and controlled by the application to said line of ringing current by the calling operator, means controlled by said first chain of tubes for applying ringing current over said toll line to the called operators end thereof, means controlled by said first chain of tubes for opening said toll line at the calling operators end thereof, and a second chain of gas-filled discharge tubes and circuit arrangements therefor associated with the called operators end of said toll line and operated by the ringing current transmitted thereover for controlling said supervisory tube.

9. In a telephone system including a calling operators toll cord, a toll line and a called operators toll cord, the combination of a supervisory gas-filled discharge tube (T44-Fig. 6) capable of giving a visual signal located in said calling operators toll cord, a sleeve circuit for said toll line at the calling toll operators end thereof having its characteristics so adjusted that when the calling operators toll cord is initially connected therewith said supervisory tube will be held unoperated, and a plurality of gas-filled discharge tubes and circuit arrangements therefor associated with said toll line and started in operation by the application of ringing current to said toll line by said called toll operator for changing the characteristics of said sleeve circuit whereby said supervisory tube may be operated.

10. In a telephone system including a calling operators toll cord, a toll line. and a called operators toll cord, the combination of a supervisory gas-filled discharge tube (T44Fig. 6) capable of operating a visual signal located in said calling operators toll cord, a sleeve circuit for said toll line at the calling operators end thereof having its characteristics so adjusted that when the calling operators toll cord is initially connected therewith said supervisory tube will be held unoperated, a second gas-filled tube (T45Fig. 7) located in said toll line at the calling operators end thereof and operated when said calling operators cord is connected to said toll line, a first chain of gas-filled discharge tubes (T47T4sFig. '7) associated with said toll line and conditioned for operation by the operation of said second tube, a second chain of gas-filled discharge tubes associated with said toll line and operated by the application to said line of ringing current by said called operator for operating said first chain of tubes previously conditioned for operation, and means controlled by the operation of said first chain of tubes for changing the characteristics of said sleeve circuit of said toll line whereby said supervisory tube will be operated.

11. In a telephone system including a subscribers circuit, a toll switching trunk, a toll operators cord circuit, the combination of supervisory signaling means located in said toll switching trunk at the subscribers end thereof, a first gas filled discharge tube (TioFig. 5) in said trunk normally released and controlled by the connection of said toll cord to said trunk, means controlled by the operation of said first tube for closing a low impedance path across the talking conductors of said trunk, a second gas filled discharge tube (T12Fig. 4) in said trunk normally released, means operable when said low impedance path is closed for operating said second tube, and means controlled by the operation of said second tube for operating said supervisory signaling means.

12. In a telephone system including a subscribers circuit, a toll cord having supervisory signaling means associated with the sleeve circuit thereof, a toll switching trunk including a first gas filled discharge tube (Tia-Fig. 4) normally released and operated when said trunk is connected to said subscribers line, a second gas filled discharge tube (T15-Fig. 4) normally released and conditioned for operation by the operatiQIl of said first tube, means controlled by the operation of the subscribers switchhook for operating said second tube, a chain of gas filled tubes in said trunk controlled by the operation of said second tube, means controlled by said chain of gas filled tubes for varying the characteristics of the sleeve circuit of said toll cord when connected to said trunk whereby said supervisory signaling means may be controlled.

13. In a telephone system including a calling operators toll cord, a toll line and a called operators toll cord, the combination of supervisory signaling means located in said toll line at the called operators position, a first chain of gas filled discharge tubes and circuit arrangements therefor associated with said toll line and controlled by the application to said line of ringing current by the calling operator, means controlled by the said first chain of tubes for applying ringing current over said toll line to the called operators end thereof, means controlled by said first chain of tubes for opening said toll line at the calling operators end thereof, and a second chain of gas filled discharge tubes and circuit arrangements therefor associated with the called operators end of said toll line and operated by the ringing current transmitted thereover for controlling said supervisory signaling means.

14. In a telephone system including a calling operators toll cord, a toll line and a called operators toll cord, the combination of supervisory signaling means located in said calling operator's toll cord, a sleeve circuit for said toll line at the calling operators end thereof having its characteristios so adjusted that when the calling operators toll cord is initially connected therewith said supervisory signaling means will be held unoperated, a first gas filled tube (TFig. 10) located in said toll line at the calling operators end thereof and operated when said calling operator's cord is connected to said toll line, a first chain of gas filled discharge tubes (T47, T4sFig. 10) associated with said toll line and conditioned for operation by the operation of said first tube, a second chain of gas filled discharge tubes associated with said toll line and operated by the application to said line of ringing current by said called operator for operating said first chain of tubes previously conditioned for operation, and means controlled by the operation of said first chain of tubes for changing the characteristics of said sleeve circuit of said toll line whereby said supervisory signaling means will be operated.

CHARLES S. DEMAREST. 

